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> current partnership with Microsoft as the sole "external provider"

I don't see that as an special partnership, MS used their public API to integrate Dropbox into their apps. The benefit is more on MS side for Office, since they invited more DB users to use MS Office.

They acquired Berlin based startup a year ago https://readmill.com/, which was building ebook reading service.

They specialised in PDF and epub formats.

Competing with MS Office... The business acumen in dropbox seem like an all-time high.